docs: VitePress with full vi/en i18n + quiz, exam library, URL shortener guides

[behitek]

Summary

Moves the docs from Docsify to VitePress with full Vietnamese and English support, and adds how-to guides for features that had no docs. plans/ (local working notes, e.g. the refactor plan) is now gitignored.

Platform

• Docsify → VitePress. The Vietnamese pages move from docs/ to src/ with the same URLs and keep their git history.
• Old Docsify links redirect: /#/site/x?id=y → /site/x#y.
• Built-in search per language, Mermaid diagrams, LCOJ logo and brand color, dark mode.
• Every page has "Edit on GitHub" and "last updated". The footer credits DMOJ/VNOJ and links support (GitHub Issues · behitek.com · luyencode.net contact).
• Branding and SEO like luyencode.net: the LCOJ favicon set (replacing the inherited VNOI icon), theme-color, Open Graph and Twitter cards with og_logo.png. Each page gets a canonical URL, vi↔en hreflang alternates, its own title and description, and there is a sitemap.xml.
• GitHub Actions deploy to Pages on push to master. CNAME (docs.luyencode.net) is kept.

i18n

• Vietnamese stays at /, and English is at /en/, with a language switcher.
• All 31 pages exist in both languages. Commands and config are not translated. UI labels match the app's own English strings.
• npm run check:locales runs in CI and fails if a page exists in only one language.

New guides (vi + en, written from the lcoj-site code, with diagrams)

Page	For
features/quiz: taking a quiz	Students: finding quizzes, integrity monitoring, question types, autosave, time limit + 30 s grace, result modes, ranking, FAQ
features/quiz_authoring: creating quizzes	Teachers: permissions, question bank, grading strategies (worked example), exact XLSX/JSON import format, quiz settings, clone, attempts/violations review, regrade
features/library: exam library	Readers (browse, flipbook) and admins (add exams, PDF storage, troubleshooting)
features/url_shortener: URL shortener	Staff (create links), admins (permissions), operators (dedicated short domain)

Fixes

• The dev server rendered a blank page because of Mermaid 11.17's CommonJS fastdom dependency. Mermaid is now pre-bundled.
• Nested code blocks broke the layout of managing_problems and management_commands (both languages).
• - [ ] checklists now render as checkboxes.

Not in this PR

• The existing pages were translated as-is, so both languages still carry the same outdated facts (env example, ports, permission descriptions, generator example, …). They get corrected together in Phase 1; tracked in the local refactor plan.
• lcoj-site bugs found while writing: the URL shortener's redirect isn't enabled, so short links 404; quiz grading edge cases; several wrong Vietnamese quiz labels. They are tracked in the local refactor plan for the app backlog.
• No screenshots, by decision. Visuals are diagrams, tables and callouts only.

⚠️ Before merging

This PR removes docs/, which the current Pages source serves from.

1. Settings → Pages → Source → GitHub Actions
2. Merge right away. The first deploy takes about a minute.

Test plan

• npm run check:locales: 31 pages in sync
• npm run build: clean, no dead links; an extra scan of the built HTML found 0 broken #anchor links
• Chrome: all 8 feature pages (vi + en) render, all Mermaid diagrams draw with no syntax errors, no JS errors; legacy /#/ redirects work in dev and preview
• CI green on this branch
• After merge: https://docs.luyencode.net and https://docs.luyencode.net/en/ load, and an old /#/... link redirects

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